Reflow soldering is used in the surface mount industries to affix electronic components onto the surface of a printed circuit board. Solder paste is first applied to the printed circuit board ("PCB"), components are then positioned on the PCB and, the printed circuit board is heated in a convectorized oven to allow the solder to liquify and come into contact with the electronic components leads and the printed circuit board lead areas. The printed circuit board is then cooled forming an electrical connection between the components and the board.
The solder paste is comprised of a metal alloy to form the electrical connection and a flux. During the reflow soldering process, traditional solder fluxes remain on the printed circuit board and must be cleaned off at a later point usually using a solvent such as organic compounds such as water or flourochlorocarbons. These materials cause waste handling concerns.
Recently, no clean, low residue soldering materials have begun to be used in the reflow soldering industry. These no clean, low residue solders require no cleaning step of the printed circuit board because they are designed to emit their flux residues, rather than have them remain on the board. Because of this nature of no clean fluxes, reflow ovens have a great deal of contaminates to deal with in their interiors. It is important that the buildup of these materials does not hinder thermal performance.
In addition, many low residue, no clean fluxes require the addition of inert gases into the atmosphere of the reflow ovens. Thus, any system designed to manage contaminates in a reflow oven atmosphere with added inert gas must take this special atmosphere into account and preserve the atmosphere of the oven as much as possible.
An additional problem of no clean flux use is that in the cooling chamber portion of the reflow oven, flux emitted into the furnace atmosphere tends to condense very quickly onto their cool surfaces. The condensation of such flux byproducts onto a traditional cooling apparatus within the reflow oven will result, over time, in the erosion of cooling performance. This, in turn, can cause solder joint reliability problems within the soldered article because of the lengthened cooling time. In order to prevent buildup in currently used cooling systems, extensive maintenance is required to constantly remove the flux condensate form the walls of the system. This is undesirable.
Present cooling systems in reflow ovens may incorporate filtration alone or a "high temperature" vaporization method to help remove flux byproducts. Unfortunately, filters only move the problem from one location to another. Flow levels still fall off and heat exchangers still clog. In addition, vaporization systems have limited value since the flux byproducts tend to contaminate the oven chamber and soldered articles with deleterious conductive carbon residues or short-chain organic greases. They also can damage cooling apparatus components, and these burnoff systems reduce cooling since the high temperatures required render heat exchangers useless. Filter systems also reduce cooling efficiency by restricting air flow in the cooling systems as the filter become filled with flux byproduct, and they require continual and extensive maintenance and monitoring to maintain correct convection flows.
One cooling system, "Cooling and Exhaust Unit for Solder Reflow System", U.S. Pat. No. 4,912,857, by Parent, et. al., is disclosed to cool and exhaust gases from the cooling zone of a reflow soldering machine. The system blows cool external, ambient air onto the printed circuit boards and exhausts gases and fumes out of the system in a balanced fashion. Such a system does not recirculate the atmosphere air of the oven and does not clean flux byproduct from the atmosphere air prior to exhaust. Some form of stack "collection" is necessary for the removal of flux byproducts and to minimize flux emissions.
An apparatus is needed to remove aerosol flux byproducts from the process gas of reflow ovens. Such an apparatus should remove the aerosol byproducts internally without the need for external stack cleaning systems and without incineration. An apparatus is needed that will clean flux byproducts without harming cooling efficiency of the apparatus over time. An apparatus is needed that will not require a great deal of maintenance or troubleshooting. An apparatus is needed whereby maintenance and cleaning can be done without causing any significant delay in continuous reflow oven production operations.